Within-subject variation of carcinoembryonic antigen in colorectal cancer -- application of reference of change values and individual reference ranges to patient follow-up.

Dynamic interpretation of results is an alternative approach to the conventional cut-off procedure. Reference change limit is a valuable reference point to interpret dynamically tumour marker values when only very few serial results can be obtained from a patient after treatment. In this paper, a reference change limit of 2.0 microg/l was estimated for carcinoembryonic antigen in patients with complete remission of colorectal cancer. This figure means that a difference greater than 2.0 microg/l has at least a chance of being statistically significant (at 0.05 probability). As complementary information to the reference change limits, with more than four successive results, a simple time series model can be used to obtain predictive limits for the next observation.